One potential area of investigation — V5R3 Save and restore spooled files.
I have no info on how that would work for you. The QUSRTOOL option must be installed and the SAVRSTSPLF tool must be created.
By “upgrade our disks“, do you mean that you intend a complete replacement of your disks, followed by an install of V5R4, followed by a restore of objects from V5R3? Or are you going to upgrade your disks while V5R3 is installed by adding drives and possibly migrating data from some older drives to newer ones? …and then upgrade to V5R4?
Is this intended to accomodate 17GB load-source?